/Engine/Source/Developer/DerivedDataCache/Public/DerivedDataBuildFunction.h
This function is only called when canceling a build that has called BeginAsyncBuild, which requires this to be implemented to return only after EndAsyncBuild has been called exactly once on the given context. Due to timing, this can be called after EndAsyncBuild, and must handle that case, but will never be called unless BeginAsyncBuild has been called.
virtual void CancelAsyncBuild ( [FBuildContext](API\Developer\DerivedDataCache\FBuildContext) & Context ) const